Pneumatic conveyance devices for agricultural applications have been known for some time. These devices commonly employ a fan or blower to create a vacuum for sucking grain or other granular material into a cyclonic receptical from one location and transferring the material through an outlet of the receptacle to a second location by means of compressed air provided by the same fan or blower. Because these pneumatic conveyance devices rely on a moving airstream to effect the transfer of granular material, many smaller or lighter particles associated with the granular material become airborne. These airborne particles tend to follow the airflow through the cyclonic receptacle to the intake of the blower. Often these airborne particles are abrasive and, therefore, cause premature wear to the air moving blower components. In agricultural applications, this excessive wear problem is especially serious when dealing with grains such as white beans or soybeans which are often grown in a sandy soil and, hence, tend to have a considerable amount of particulate sand associated therewith. However, all types of grains have a certain amount of foreign material such as chaff and weed seeds which tend to follow the airstream through the blower components.
Because of the large volumes of air used by these pneumatic devices, it is not practical to use a conventional filter to remove unwanted airborne particles from the air entering the blower. A conventional filter would rapidly become clogged and restrict airflow to the blower. Cyclone arrangements have been used to separate particulate matter from the airstream leaving a collecting receptacle of a device of this type. The particles are forced to the walls of the cyclone and gravitationally fall to a receptacle out of the airstream.
For example, U.S. Pat. No. 3,955,236 dated May 11, 1976, describes a vacuum collector system for gravel, wherein the gravel is sucked into a first compartment, the outlet of which is connected to a cyclone separator for removing dust from the airstream entering the pump. Also, U.S. Pat. No. 4,218,226 dated Aug. 19, 1980, describes a vacuum apparatus for particulate matter having a similar arrangement of components including a cyclone for cleaning the air entering the fan.
It has been found that the use of a single cyclone to clean air being drawn into a blower of a pneumatic conveyance apparatus is often not effective to remove sufficient abrasive particulate matter to prevent premature wear to the blower components. This is especially true in those devices employing a blower which comprises a high pressure positive displacement air pump having air moving components comprising precisely machined interengaging surfaces.
More recently, a cyclone arrangement described in U.S. Pat. No. 4,599,016 dated July 8, 1986 has been used to effectively remove a larger amount of the airborne particles from the airstream leaving the collecting receptacle. This cyclone arrangement includes a primary cyclone and a secondary cyclonic air cleaner wherein the secondary cleaner is positioned inside the primary cyclone and is provided with its own collecting container. The main problems experienced with this secondary cyclonic air cleaner are the limited capacity of the collecting container and the increased restriction in the airstream due to cyclonic airflow. When it is desired to move a large quantity of grain having excessive foreign material with this cyclone arrangement, it may be necessary to interrupt the operation repeatedly in order to empty the secondary collecting container.